This invention relates to furnaces for melting thermoplastic material, such as glass, and more particularly to an electric furnace in which the electrodes at different levels are immersed to different depths.
In vertical electric furnaces, batch is fed over a large part of the top surface, melted and refined in upper and lower zones and removed from an outlet near the bottom. Wall electrodes and deeply immersed electrodes have been used.
U.S. Pat. No. 2,993,079 -- Augsburger is an example of a shallow vertical furnace, while U.S. Pat. No. 3,524,206 -- Boettner et al. is an example of a deep vertical furnace.
U.S. Pat. No. 3,583,861 -- Preston is an example of a vertical furnace employing different levels of tiers of electrodes, some of which are staggered with respect to others. The aforesaid Boettner et al. furnace employs wall electrodes whereas U.S. Pat. No. 3,742,111 -- Pieper employs deeply immersed electrodes. Auxilliary wall electrodes have been used with deeply immersed main electrodes to improve furnace start-up.
In all of the foregoing furnace, the size, shape and stability of the fusion zone (defined below) are critical, but electrode immersion has not heretofore been used to control the fusion zone and convection pattern.
The fusion zone may be defined as the interface between the glass batch raw materials and the molten glass where the raw materials are converted into glass and carried into the molten glass by the convection currents.
For simplicity, consider a furnace at low pull that has a relatively flat fusion zone and no hot spots on the flat top batch blanket surface. The local batch fill on this blanket would correspond to the local melting rate from the fusion zone if material moves vertically down within the blanket. The top surface is relatively cool. As one moves down into the blanket, the temperature begins to rise gradually -- rising to perhaps about 200.degree. C. in the first several inches. In this zone, the only significant change that takes place is evaporation of moisture in the raw materials. At deeper blanket layers where appropriate temperatures are reached, solid-solid chemical reactions will occur. The gases released at this elevation can rise through the porous, unreacted, raw material layers. A portion of these gases might condense if temperatures in the blanket are below their dew points but given a stable blanket, the major portion (nitrogen, carbon dioxide, etc.) escapes. The presence of gases in the blanket serves to enhance its insulating characteristics. At still deeper layers within the blanket the temperature rises rapidly by comparison. Low melting compounds begin to melt releasing even more gas that escapes through the blanket.
The sticky, glassy layer with a very large temperature gradient which exists in the last inch or two is the fusion zone. This layer contains gaseous inclusions that decrease in number and size as the molten glass is approached. Thus in a stable blanket, a variation of physical characteristics exists as the temperature increases first slowly and then rapidly. The blanket is quite fluffy and porous near the top. It then turns into sintered solid mass. A part of this solid mass turns into a semi-molten low viscosity liquid and then into a sticky viscous glassy mass. The density of the blanket increases with depth. The non-condensable gases can escape freely through the porous blanket but only with difficulty and perhaps only partially through the molten glassy phase. This gradual change in batch blanket is extremely important in determining the residual gaseous inclusions.
For example, when a hot spot occurs in the blanket, this gradual progression is destroyed for all practical purposes. If one continues to fill over the hot spots, the evolved gases will find it difficult to escape. The hot spots will act as a virtual trap for the gas. The stable cold blanket, by contrast, allows the orderly escape of gases through it.
A similar but somewhat less obvious situation exists for the dissolution of sand grains. At elevations in the blanket where temperatures are correct, the fluxes are melted into a low viscosity liquid. It is the chemical reaction between this liquid and sand grains that produces alkali silicates. It is extremely important that liquid fluxes are given sufficient time to react with sand grains resulting in the first formation of glass. If this chemical environment is not maintained, the dissolution of sand grains by alkalies will be slowed resulting in poor quality glass leaving the batch blanket thus increasing the workload on the bath. A stable blanket "naturally" provides the time necessary for silica grain dissolution. This is because in the stable blanket, the sticky, viscous, glassy layer prevents the low viscosity molten fluxes from "running" and depleting the environment of the chemicals necessary for grain dissolution. This phenomenon has been called overmelting and also occurs in gas fired furnaces when the temperatures are extremely high. Thus, the stability of batch blanket is crucial in preventing the gaseous inclusions and unmelted silica in the glass from leaving the fusion zone.
The fusion zone at the center of the furnace sinks lower into the furnace as the pull is increased. This reduces the residence time of glass sheared off from the lower section of the fusion zone, resulting in decreased melting capacity or impaired quality of glass produced. Also, as pull is increased, the power input to the furnace must be increased, resulting in higher power concentration around the electrodes. This causes higher temperature near the walls and the electrodes, resulting in higher heat losses, shorter furnace life, increased electrode wear and instability of the fusion zone further adversely affecting glass residence time and quality.
The instability of the fusion zone is related to the resistivity/temperature relationship for the glass. The molten glass acts as an ohmic resistance that decreases as the temperature is increased. For a fixed applied voltage across the circuit, the power generated increases as resistance is decreased, i.e., when temperature is increased. When a temperature gradient exists from the walls to the center of the furnace near the elevation of the fusion zone, the local power dissipated is not uniform from wall to center. The higher the temperature in a local zone, the more highly disproportionate its share of power will be. This causes the local temperature to rise further which in turn decreases local resistance thereby increasing local power dissipation further. A potentially unstable situation exists. This causes local hot spots on the blanket top and may cuase severe instability and even total melting of batch blanket for a glass with a steep resistivity-temperature relationship. Likewise, a high infra-red absorption glass will cause a fusion zone instability by increasing the temperature differences between furnace center and wall due to reduced radiation heat transfer. The instability tendency increases as pull is increased.
In addition to their effect on the stability of the fusion zone, convection currents strongly affect the quality of glass. With equal immersion of electrodes at all levels, a good control of either wall or furnace central convection currents is obtained. However, a simultaneous control of the wall and center convection currents is not possible even with staggered electrodes of equal immersion.
In an electric furnace, physical differences such as electrode immersion, electrode tier arrangement, power distribution and batch fill pattern have a major influence on such things as fusion zone shape and stability, convection current patterns and temperature distribution. These are in turn related to economic benefits such as furnace capacity, furnace and electrode life and glass quality.
The stability of the blanket and the fusion zone are particularly important in determining the glass quality and the furnace capacity and therefore a method of obtaining more uniform temperatures across the fusion zone, of keeping the fusion zone high in the unit and of simultaneously controlling wall and center convection currents in the furnace would achieve the objectives of superior glass quality and greater furnace capacity.